1. Field of the Invention
The present invention relates to an exhaust emission control device suitably used for a diesel engine.
2. Description of the Related Art
An exhaust gas from the diesel engine contains the particulate (hereinafter referred to as soot) mainly composed of carbon. Various techniques for suppressing the soot to be exhausted into the atmosphere have been put into practical use.
As one of the techniques, a continuous regeneration exhaust emission control device is well known in which an oxidation catalyst and a particulate capturing filter (abbreviated as DPF) are disposed in the order from the upstream side. Another device is also well known in which alkaline metal catalyst is held in the DPF to improve the regeneration performance of the DPF.
However, the alkaline metal catalyst causes depression in a high temperature area owing to reconnection of alkaline metal and splashing by evaporation, whereby it is necessary to suppress an excessive rise in exhaust gas temperature while driving fast or driving at high load.
By the way, an engine with a capacity fixed turbo charger can suppress an exhaust gas temperature during the high rotation and high load driving by setting an injection valve opening pressure of a waste gate valve at a higher value to raise a supercharging pressure and increase an intake air flow.
However, when the injection valve opening pressure of the waste gate valve is raised, the supercharging pressure in a low rotation area of the engine also rises. Therefore, if an internal pressure of cylinder of the engine is suppressed within a permissible limit, it is required that the injection timing of fuel is retarded, resulting in a problem that the fuel consumption is worse. Also, a pumping loss in the low rotation area is increased, leading to a risk that the fuel consumption is also worse.